Fire detection systems comprising a fire detector or a plurality of fire detectors having different response thresholds are generally well-known in the art. They comprise fire sensor elements which generate an output signal in response to fire or fire-related phenomena and evaluating circuit elements having a threshold detector with a predetermined threshold value which thereby establishes the sensitivity of the detection system. When the output signal from the sensor exceeds a predetermined threshold value, an alarm signal is generated. In order to detect fires in their earliest stage, fire detectors generally rely upon the use of a threshold detector with a low threshold value. Nevertheless, systems utilizing fire detectors with low threshold values (high sensitivity) have the drawback that a fire alarm will be activated in response to benign fire phenomena even of short duration.
To remedy this drawback, Swiss patent No. 629,905 discloses a fire and/or gas sensing and signaling system wherein the signaling portion of the system has a warning indicator and an alarm indicator. The warning indicator provides a warning output if fire phenomena above a first, lower threshold is sensed by the detector. An alarm indicator provides an alarm output if the fire phenomena sensed by the detector exceeds a second and higher threshold. The signalling system may further comprise timing stages which start a timing interval upon sensing fire phenomena above the first and lower warning threshold value. If the warning signal above the first threshold level persists for the duration of a time interval predetermined by the timing stage, that signal will lock-in and be indicated in the control panel. If the fire phenomena sensed by the detector exceeds the second, higher threshold level, an alarm signal will be activated. Timing stages, although shorter than those for the warning signal, may be used in connection with the alarm signal. The objective of such a prior art system is to be able to distinguish positively between a fire condition lasting for only a short time and genuine fire condition necessitating an alarm.
British patent No. A 2,043,977 discloses a fire detection system which detects different types of fires, e.g. fires involving flammable liquids and smoldering fires. In this detection system, sensor elements which respond to various different kinds of fire phenomena are combined using an "OR-logic" in the evaluating circuit. These systems, however, are particularly prone to false alarms.
Fire detection systems are also known which utilize a combination of sensor elements with an "AND-logic". See for example Swiss patent No. A 506,147. These systems however reduce the overall sensitivity of detection since an alarm signal is only generated if both sensor elements respond simultaneously. Swiss patent No. 572,252 sought to remedy this drawback, however, by designing the electronic evaluating circuit in such a way that the fire sensitivity of one sensor would be increased if the other fire sensor is activated by a fire phenomena. European Patent No. A 0,076,338 discloses similar evaluating circuitry for a fire detection system.
Although it would seem a simple matter to reduce the sensitivity of the fire detectors in a system and thereby reduce the incidence of false alarms, this is not an acceptable solution since doing so defeats the primary purpose of effective early detection of fire conditions. Accordingly, the object of the present invention is to provide a fire detection system that avoids the drawbacks of false alarms as well as the drawbacks of the prior art systems which have sought to minimize the incidence of false alarms, without simultaneously reducing the sensitivity of the fire detector.